Mapping and map-based cloning

Development of a genetic system and genetic resources is one of the prerequisites for establishing Lotus japonicus as a model legume. Successful identification and characterisation of several key plant genes controlling symbiosis with nitrogen fixing bacteria and mycorrhizal fungi demonstrate the potential of this approach. We provide here an overview of efforts to establish a genetic map and approaches using genetic marker information for positional cloning. In L japonicus six independent mutant populations in the ecotype Gifu were obtained with EMS, fast neutrons, T-DNA, transposon, or tissue culture. More than 500 independent symbiotic mutants have been isolated and at least 24 independent loci found. General mapping populations are available from crosses between ecotype Gifu and ecotype MG-20 and between Gifu and L filicaulis. These parents are also used for establishing populations for map-based cloning. Five BAC and TAC libraries were constructed. The general strategy for map-based cloning in L japonicus is described and map positions of eleven sym genes are shown. Four L japonicus sym genes have been isolated using map-based cloning and two genes have been isolated with transposon tagging.